Oil shale, tar sands (also called oil sands), extra heavy oil, distillation bottoms, coal tailings, and lignite (a group which collectively will be referred to as “unconventional oil source materials” herein) offer a resource for meeting the world's demand for hydrocarbons as conventional, low viscosity petroleum runs out. High viscosity and a heavy burden of mineral solids such as sand and clay have made unconventional oil source material difficult to process. Conventional methods of oil extraction can carry a high environmental cost on two accounts: CO2 from the natural gas burned to heat the water for extraction, and water pollution from the discharge of the residue, liquid and solid, from processing.
Capturing and sequestering CO2 emissions (CCS) from tar sands processing (which are half of all of Canada's CO2 emissions) is not feasible at the scale required, given the present state of the art of CCS. Recent reports of leaks in Canadian sequestration projects, and the huge water requirements of chemical CO2 capture, make the need for an alternative to the hot water extraction method of tar sands processing imperative.
Tailings ponds (sludge ponds) from conventional oil extraction are a growing toxic dump covering over fifty square kilometers in the boreal forest of Alberta, Canada, a blight now easily visible from space. Six barrels of aqueous tailings are produced for each barrel of oil extracted from the Athabasca tar sand. These tailings ponds are mostly a gravy-like thick suspension of fines, which will not settle. The top layer of clear water is poisoned by naphthenic acids, which are deadly to migrating waterfowl which land in the sludge ponds. The conventional means for addressing the bird kill problem is to try to frighten the birds when they approach. Water pollution from sludge ponds also poses a health risk to humans because the poisons migrate into the water supply.
After 40 years, as of 2007 no reclamation certificate for a sludge pond had been issued by the government of Alberta. Despite public clamor, there is no effective remediation plan in place for the sludge ponds, so the first task is to prevent them from getting worse. The need is clear for an alternative to the heavy consumption of fresh water involved in current extraction methods. For a description of the wasteful conventional art, see Chrones, et al. “Bitumen and Heavy Oil Upgrading in Canada,” Petroleum Science and Technology, 7:5, 783-821 (1989).
Naphthas are the petroleum fraction having a boiling point below 200° C. Oils are higher boiling point products. Naphthenic acids poison the sludge ponds because devolatilization of the residual solids is incomplete. Injected steam will quickly lose enthalpy uselessly heating the water and solids which are associated with the desired hydrocarbons, so devolatilization of the residue is incomplete due to the inherent limitations of any steam extraction process.
Shear thinning, or mechanical reduction of viscosity, is a well-known phenomenon in rheology. Visbreaking, or thermal reduction of viscosity, is conventionally practiced by heat transfer from external fuel combustion. Heat transfer is either quickly (coil-type) or slowly (soaker) in a vessel which contains the material. Rotating the vessel to tumble the feedstock is known to the art.
Possible mechanical visbreaking was observed in the small batch experiment reported in McKay, et al. “Fundamentals of Mechanical Upgrading of Athabasca Oil Sands: Mechanisms of Sand and Bitumen Separation,” Separation Science and Technology, 16:3, 237-252 (1981), but the authors did not follow up or claim such a result.
The composition of tar sands typically is 85% silica sand, clay, and silt, 5% water, and 10% bitumen. The average grain size of the sand is 0.5 mm diameter, and 1.5-20% of the sand is free of bitumen. Sand density is 2.65 g/cm3 and bitumen is 1.01 g/cm3 The intrinsic water content of tar sands is an unexploited resource, and the present disclosure is directed to using this intrinsic water content instead of water withdrawals from the environment to generate hot water for oil extraction.
The conventional method for oil extraction from tar sand is the Clark Hot Water Extraction (CHWE) process developed nearly a century ago. Excavated tar sand is comminuted for size reduction and hot water at 50-80° C. is added, resulting in a slurry out of which bitumen is separated as a froth comprising 60% bitumen, 30% water, and 10% solids by weight. Approximately 70 weight percent (83.5 volume percent) of the bitumen in the ore is recovered as synthetic crude.
Separating bitumen from the sand by mechanical means has not progressed in the 30 years since the pioneering research in the field. See McKay, et al. “Fundamentals of Mechanical Upgrading of Athabasca Oil Sands: Mechanisms of Sand and Bitumen Separation,” Separation Science and Technology, 16:3, 237-252 (1981), and Grant, et al. “Mechanical Upgrading of Athabasca Tar Sands in a Rotating Contactor,” Ind. Eng. Chem. Process Des. Dev., 19:1, 185-189 (1980). These batch experimental devices used a rotor-stator mill in a cold water process to chip off bitumen to produce free sand, and shear heating of the bitumen to the point of adhesion with the stator was dismissed as an unwanted experimental artifact. The upgrading in the prior art occurs prior to retorting.
Thermal recovery processes heat the separated bitumen froth in a retort to processing temperatures of 450-550° C. and extract from the retort a product stream comprising gases, naphthas (boiling point below 200° C.), and oils. Solids in the froth drop out of the retort and are further heated to about 550-600° C. by combustion of their residual carbonaceous compounds during a combustion step in a sand loop outside of the retort. Recycling the hot sand from the sand loop into the retort supplies heat for the pyrolysis step.
Oil shale, like tar sand, has a heavy burden of mineral solids. Kerogens (solid high molecular weight hydrocarbons in the sedimentary rock) have high boiling points and the mineral solids and water associated with the kerogens in the oil shale deposit act as parasitic heat sinks, making in situ steam extraction difficult.
Coal tailings are a mixture of coal and minerals which may result from mining of the various grades of coal, such as anthracite, bituminous coal, and lignite. Lignite is a low-grade coal comprising a high mineral content. Slag from combustion of lignite is collected in ash ponds, which are a growing blight near coal-fired power plants. Transportation of lignite to the coal plants is wasteful to the extent that minerals are being transported as well as hydrocarbons. Extracting oil or syngas from the lignite near the mine would save on transportation costs and avoid the ash pond problem.
Char mixed with soil is called terra preta. Long experience with terra preta in the Amazon proves that biochar is excellent for agriculture. Completing the pyrolysis of tar sands for complete residue conversion would make the residue of tar sands oil extraction a beneficial product instead of toxic waste.
The present disclosure is directed to the long-felt but unmet need for a continuous and low-tech method for making char sands from tar sands, without CO2 emissions from heating water or burning residue, and with minimal wastewater discharges to the environment.
Other uses for the disclosure will be obvious to the skilled practitioner, including the processing of other feedstocks where mineral burden interferes with the proper extraction of the product, such as ore processing in other mining operations.